The present invention concerns a method of making a nuclear fuel pellet for a nuclear power reactor.
Different manners of producing nuclear fuel pellets are known by a person skilled in the art. It is normal to make the nuclear fuel pellet from a nuclear fuel material in powder form. The nuclear fuel material may for example be UO2, where U is enriched with regard to 235U. The powder material may also include additives, such as U3O8 and binder material. The powder is pressed in order to form a so-called green pellet. The concept “green pellet” in this technical field means the pressed pellet before it is sintered. The green pellet is thus thereafter sintered in a furnace. The sintered pellets are thereafter ground in order to obtain the correct diameter and surface finish.
It is also known to include some additives in the powder in order to increase the grain size in the sintered pellet. For example WO 00/49621 A1 gives some examples of such additives and describes how the nuclear fuel pellet may be produced.
Other examples of how to increase the grain size in the nuclear fuel are described in GB 2177249 A, GB 2020641 A, GB 2107691 A and DE 3235944 A1.
WO 2005/041208 A2 describes that a porous uranium dioxide arrangement is infiltrated with a precursor liquid in the form of allylhydridopolycarbosilane in order to enhance the thermal conductivity in the nuclear fuel.
Another phenomenon that occurs when using nuclear fuel in a nuclear reactor is a structure in the used nuclear fuel pellets called high burn-up structure (HBS) or rim structure. When the nuclear fuel has been used for a longer time in a nuclear reactor (i.e. a high burn-up) a new restructured configuration appears at the outer thin region of the fuel pellet. This phenomenon is described for example in the article “The high burn-up structure in nuclear fuel” by V. V. Rondinella et al. in Materials Today, December 2010, Volume 13, No. 12, pages 24-32. The HBS means that the grains in the outer region of the nuclear fuel pellet subdivide into very small grains. The outer region in which the HBS appears may for example be less than 100 μm thick. Different problems caused by the HBS are mentioned in this document.
WO 97/13252 A1 and JP 9-127279 A describe different ways of reducing problems of the rim structure.